The present invention relates to a scanning electron microscope used to observe fine structures such as those of semiconductor devices. More particularly, the present invention relates to a scanning electron microscope having a structure which makes it convenient for an operator to control an observation field which is set on a sample.
In a conventional scanning electron microscope, an electron beam is emitted from an electron gun, converged by a converging lens and deflected by deflection coils. The deflected electron beam is passed through an objective lens to scan the surface of a sample in a two-dimensional manner. Then, secondary electrons emitted from the sample surface are converted into an image signal through a secondary electron detector or other similar detector, and a magnified image of the sample is displayed on the screen of an image display device (e.g., a CRT display) on the basis of the image signal.
The observation magnification can be set to a desired value within a predetermined range by the operator, and the electron beam scanning range, scanning speed, etc. on the sample are adjusted in accordance with the set observation magnification. A range that is specified as an observation object by the operator within the electron beam scanning range on the sample is an "observation field", and an image of the sample in the observation field which is magnified at the set observation magnification is displayed on the screen of the image display device.
There are two methods for moving the observation field on the sample to thereby move the observation image of the sample on the screen of the image display device: a mechanical field moving method in which a stage on which a sample is placed is moved by a movement controller; and an electric field moving method in which the electron beam scanning range on the sample is moved by an electrooptical system control circuit, or an image data selecting region in the scanning range is moved, thereby moving the observation field.
The electric field moving method has the advantage that the image can be moved at a constant speed when the observation magnification is high. On the other hand, the electric field moving method suffers from the disadvantage that the oscillation width of the electron beam is limited to a value which is much smaller than the amount of movement of the stage. The mechanical field moving method, in which the stage is mechanically moved, has the advantage that the observation field can be continuously moved, for example, to a region on the sample which is away from a region which is presently scanned with the electron beam. Therefore, the operator generally uses both the mechanical field moving method and the electric field moving method by changing them from one to the other according to need. A device which enables the two methods to be automatically changed according to the observation magnification has also heretofore been used. In this regard, as a conventional stage driving system for mechanically moving the observation field, a feed screw system has heretofore been used in which the stage is fed by driving a feed screw with a pulse motor (i.e. stepping motor) which is driven by a full-step drive control method (or by a half-step drive control method).
As a mechanical field moving method, a position control method has heretofore mainly been used in which a distance by which the observation field is to be moved (hereinafter referred to as "moving distance of the observation field") is calculated according to the amount of rotation of a track ball, and the observation field is moved on the sample by the calculated moving distance in a direction corresponding to the direction of rotation of the track ball. The track ball is a tool in which a rotating member that is buried in an operator console is rotated two-dimensionally, thereby causing pulse signals to be generated from two rotary encoders.
Another device that is used in the mechanical field moving method is a joy stick. The joy stick is a tool in which a shaft for control is rotated two-dimensionally to change output values of two potentiometers, thereby setting a direction of movement and a speed of movement in a two-dimensional plane.
The above-described conventional techniques suffer, however, from the following problems: Since there is backlash in a feed screw that is driven to move the stage, when the moving direction of the stage is reversed, such a phenomenon is likely to occur that, although the pulse motor rotates, the stage will not move for a short while, or that the stage slightly moves in a direction reverse to the direction in which it should move.
The above-described undesired phenomenon is particularly remarkable when the operator moves the stage by open-loop control while viewing the sample observation image displayed on the image display device. Thus, a sense of incongruity is caused, and it is difficult for an operator to control the stage accurately.
Further, to rotate the observation field on the sample, the conventional practice is to tilt the electron beam scanning direction. However, if the stage is moved in order to observe a region which is adjacent to the observation field in a state where the image displayed on the image display device has been rotated, since the moving direction of the stage is determined on the basis of a coordinate system used before the rotation of the observation field, the sample observation image moves in a diagonal direction and is therefore difficult to observe. For example, if the observation field is controlled so as to move toward the right or left side, with the observation field rotated clockwise through 45.degree. from the X-axis, since the stage moves in the direction X, the sample observation image displayed on the image display device moves in a direction which is counterclockwise tilted at 45.degree. with respect to the X-axis.
The conventional scanning electron microscope further involves the problem that, when the operator moves the observation field by the mechanical field moving method, that is, by mechanically driving the stage, while viewing the sample observation image displayed on the image display device, since the sample observation image moving speed varies with the observation magnification, a sense of incongruity results when moving the sample observation image, and it is difficult to place the observation field in a desired position. Particularly, when the observation magnification is high, the sample observation image moves at high speed, and therefore, it is extremely difficult to place the observation field in a desired position.
Further, the conventional scanning electron microscope suffers from the problem that, since a feed screw (e.g. ball screw), which is driven to move the stage, involves a pitch error, it is difficult to put the observation field in a desired position accurately by open-loop control. That is, even if the pitch error of the feed screw falls within the manufacturers' specifications, the positioning accuracy required for the stage may be more stringent than the specifications. In such a case, the above-described problem arises. Particularly, when the observation magnification is high, the effective observation field range, which corresponds to the effective display area of the image display device, becomes extremely narrow, and the positioning accuracy required for the stage becomes extremely high. Thus, the feed screw pitch error must be taken into account.
In view of the above-described circumstances, a first object of the present invention is to provide a scanning electron microscope which enables an observation field on a sample to be moved to a desired position by a mechanical field moving method even if there is backlash in a stage for positioning the sample.
A second object of the present invention is to provide a scanning electron microscope in which, when an observation field on a sample is to be moved toward a neighboring region by a mechanical field moving method in a state where the observation field has been electrically rotated, it is possible to move the observation field independently of the angle of rotation of it while viewing a sample observation image displayed on an image display device, and in which positioning of the observation field is easy.
A third object of the present invention is to provide a scanning electron microscope in which, when an observation field on a sample is to be moved by a mechanical field moving method on the basis of a sample observation image displayed on an image display device, the sample observation image can be moved at a constant speed on the image display device independently of the sample observation magnification no matter how high it is.
A fourth object of the present invention is to provide a scanning electron microscope in which, even if there is a pitch error in a feed screw of a stage for positioning a sample, an observation field on the sample can be accurately moved to a desired position by a mechanical field moving method.
Meanwhile, if a method in which the sample is manually moved through the stage is used, the observation field can be readily moved in a wider range than in the case of a method in which the observation field is electrically changed over. However, the manual stage moving method suffers from the problem that a great deal of time is required to search for a desired image because the image continuously moves. Further, when it is desired to display an image which is adjacent to the presently displayed image on the screen, for example, it is difficult to make the stage come to rest accurately at a point of time when the neighboring image is displayed on the screen. Therefore, the operator is likely to pass the desired image or lose sight of it.
Particularly, when the observation magnification of the scanning electron microscope is high, since the image moves at high speed in response to the drive of the stage, it is extremely difficult to allow the observation field to come to rest at the desired position.
In view of the above-described circumstances, a fifth object of the present invention is to provide a scanning electron microscope in which, when the observation field is to be moved by moving the sample, the observation field can be moved to a region on the sample which is adjacent to the present observation field at high speed and with high accuracy even if the observation magnification is high.
Regarding the above-described conventional techniques, when a sample as an object of observation is a regularly arranged device such as a semiconductor memory, it is sometimes desired to fix the moving direction of the observation field on the sample in a predetermined direction. However, when a joy stick or a track ball is used to input an observation field moving direction, since it is difficult to accurately rotate the control shaft or the rotating member only in a predetermined direction, it is difficult to move the observation field only in a predetermined direction by manual operation.
When the moving distance of the observation field is short, the amount of rotation required for the rotating member of the track ball is relatively small. Therefore, controllability is good. However, when a position on the sample which it is desired to observe is distant from the present observation field, and hence the moving distance of the observation field is long, the amount of rotation of the track ball must be increased, causing controllability to be degraded.
Particularly, in the case of the track ball, the rotational angle of the rotating member is not limited, unlike the joy stick, and the rotating member is isotropic. Therefore, it is extremely difficult to accurately rotate the rotating member only in a predetermined direction.
In view of the above-described circumstances, a further object of the present invention is to provide a scanning electron microscope in which an observation field on a sample can be accurately moved in a predetermined direction even when an input device such as a track ball is used (a sixth object of the present invention), and in which the observation field on the sample can be readily moved to either a near position or a distant position (a seventh object of the present invention).